Acquiring a small target can be challenging and generally results in long targeting times and high error rates. This is especially true when the target is clustered with other targets. One technique designed to help users acquire small targets, that is useful when targets are uniformly distributed on the screen, is the bubble cursor technique. In this technique, a cursor snaps to the closest target. There is a bubble around the cursor that varies in size such that it contains the closest target. Employing a bubble cursor may be considered a target expansion technique. This becomes clear when one labels each pixel on the screen according to which target will be acquired if the pixel is selected (clicked on) with an input device. This resulting partitioning of the screen space is also referred to as a Voronoi tessellation. The bubble cursor target expansion technique is beneficial for users. Instead of having to aim for a small target, a user can click anywhere inside the tile containing the target. This approach reduces targeting time for layouts of uniformly distributed targets.
Unfortunately, in real-world applications uniform distributions are an exception rather than the norm. Locally dense clusters of targets emerge for a variety of reasons. A user interface may represent a real-world geometry with a non-uniform structure, such as cities on a map. In other cases, it is users who manually create clusters, for example, when grouping icons on their desktops or when organizing links inside a web page. Or clusters may merge from the structure of visualized data, or may appear from targets being input into a system such as would be the case in an air traffic control system and display.
When applied to a cluster of targets, a bubble cursor shows little effect. Targets located inside a cluster are surrounded by little empty screen space. As a result, the tiles generated by the expansion are small—associated targets remain hard to acquire. When used on a device with imprecise input, such as a touch-screen kiosk, the acquisition of such targets will be error prone. The same holds true for a pen input.
Limitations in handling target clusters are not unique to the bubble cursor target expansion technique, but faced by all target expansion techniques. Some of them even impact performance negatively if applied to target clusters. Interactions between closely adjacent expanding targets sometimes cause targets to “escape” from the user.